Subduction is the key geological process that can link the Earth's surface system with deep Earth's interior. It plays a significant role for the study of deep carbon cycle. Carbon in the subducting lithosphere is present in sediments, altered oceanic crust and serpentinite. The evolution of the carbonate-bearing rocks during subduction metamorphism controls the transformation of the carbonate minerals and carbon transport process. In this study, phase equilibria were modeled for the altered oceanic crust to investigate the metamorphic evolution of the carbonate minerals in the metabasalt. The calculated results indicate that the transformation of carbonate minerals are also affected by temperature and iron content in the metabasalt, in addition of the controlling role of pressure. Phase equilibrium modeling demonstrates that carbonates in the subducted oceanic crust may experience calcite/aragonite-dolomite-magnesite transition with increasing pressure, but under HP/UHP conditions, magnesite can be transformed to dolomite with increasing temperature. The iron content in the carbonates is affected by the iron content in the metabasalt, where the iron content in the dolomite and magnesite will increase with the increasing iron content in the metabasalt. When the altered oceanic crust is under water-unsaturated conditions, almost no decarbonation will happen when the oceanic crust subducts to sub-arc depth along either low temperature or high temperature geotherm. However, under water-saturated conditions, almost all of the carbon in the metabasalt will be released when the oceanic crust subducts to sub-arc depth along high temperature or Costa Rica geotherm. The water content in the metabasalt will facilitate the decarbonation reactions.